Technical Field
Embodiments of the invention relate generally to enhancing quality of images obtained from MR scanners, including PET-MR scanners. Particular embodiments relate to formers used for positioning MR coils adjacent patient anatomy.
Discussion of Art
Magnetic resonance imaging (“MR”) uses RF antenna coils to detect rotating (“relaxation”) magnetic fields that are produced by nuclei that have odd atomic numbers, i.e., total number of neutrons and protons is not divisible by two, in response to alternately imposing and removing a scanning exciting magnetic field. Typically, MR is accomplished by imposing on a target material a strong (e.g., 1.5, 3 or 7 Tesla) constant magnetic field (“alignment field”) that spin-aligns nuclei of atoms within the target material. In order to obtain the relaxation magnetic field, an exciting field that fluctuates at radio frequencies (e.g., anywhere from about 64 MHz to about 900 MHz, typically at 1.5 T) is superimposed over the alignment field.
MR is frequently used for differentiating tissue types within a patient, and is also used for identifying fine detail structures. Typically, different pulse sequences are used for tissue differentiation. For example, a T1 pulse sequence can be used to obtain images with water appearing darker and fat brighter. On the other hand, a T2 pulse sequence can be used to obtain an image with fat darker, and water lighter.
One advantage of MR is that magnetic fields do not attenuate in body tissues, so that nucleus location can be determined (using Fourier analysis) based solely on frequency shifting between the imposed magnetic field and the response field. Another advantage is that by careful selection of pulse sequence, distinct tissues or materials can be highlighted.
For enhanced MR imaging, the detector coils typically are disposed as close as possible to the tissues being imaged. Typically, a “former” is provided both for positioning the detector coils and for positioning a patient within the MR field. Formers conventionally have been made of non-ferromagnetic materials that do not interfere with or mimic the MR response of patient tissues, and that exhibit high rigidity in a thin section (e.g., injection-molded polycarbonate or polyurethane). Such materials typically have been formed into a hollow shell, with the detector coils supported by the negative (non-patient-facing) surfaces of the former, inside the shell, and connected with ancillary circuitry housed within the shell. By locating the detector coils at the negative surface, it is possible to provide maximal room for assembly of detector coils and ancillary circuitry, enhance patient comfort, and avoid mechanical design complications associated with through-holes in the former surface. provide a flame barrier, Additionally, detector coils frequently couple and dissipate relatively large electrical power—often in excess of 15 W. By anchoring the detector coils at the “negative” (non-patient-facing) surface of the former, the former itself can then provides a flame barrier rated “V2” or higher (per UL standard 94) between the patient and the high-power detector coils. For example, a former may be fabricated to include greater than about 1.1 mm thickness of “Lexane 940”™ polycarbonate.
Although the conventional “shell” formers do provide maximal volume for ancillary circuitry, and facilitate connection of the detector coils to the circuitry, actually arranging and attaching the detector coils onto the negative surfaces has been identified as a manufacturing challenge. Partly because permanent adhesives are used for secure positioning, the negative surface coil arrays also are hard to repair. Nonetheless, many factors inform the conventional approach to building internal or negative-surface coil arrays.
MR can be combined with positron emission tomography (PET) within a single apparatus (a “PET-MR scanner”). Such scanners use one or more rings of scintillators or other detectors to generate electrical signals from gamma rays (photon pairs) that are produced from the recombination of electrons, within a target material, and positrons, emitted from decay of a radionuclide packaged in a tracer compound. Typically, recombination events occur within about 1 mm from the radionuclide decay event, and the recombination photons are emitted in opposite directions to arrive at different detectors. Paired photon arrivals that occur within a detection window (usually less than a few nanoseconds apart) are counted as indicating a recombination event, and, on this basis, computed tomography algorithms are applied to the scintillator position and detection data in order to locate the various recombination events, thereby producing three-dimensional images of the tracer disposition within the target material.
Typically, the target material is body tissue, the tracer compound is a liquid analogue to a biologic fluid, and the radionuclide is disposed primarily in body tissues that make use of the biologic fluid. For example, a common form of PET makes use of fludeoxyglucose (18F), which is analogous to glucose with the 18F radionuclide substituted for one of the hydroxyl groups ordinarily composing glucose. Fludeoxyglucose is preferentially absorbed by brain matter, by the kidneys, and by growing cells (e.g., metastasizing cancer cells). Thus, PET is frequently used for oncologic studies, for localizing particular organs, and for studying metabolic processes.
PET signal attenuation is very specific to patient anatomy and material properties. In particular, the dense and rigid polymers typically used in MR formers (e.g., polycarbonate, polyurethane) can significantly attenuate the gamma rays generated by positron recombination events. For example, a conventional head/neck coil former built from polycarbonate may attenuate PET sensitivity by as much as about 21 percent. Moreover, dense polymers tend to be absorptive of positrons, producing noisy recombinations that can obscure PET imaging of body tissues. However, given the conventional preference for negative surface coil and circuit assemblies, polycarbonate, polyurethane, and similar high-density, high-rigidity polymers have been seen as the typical materials for building coil formers.
One factor particularly promoting the retention of high-density polymers has been that lower-density materials would require increased shell wall thicknesses, thereby displacing coils from the patient and diminishing MR signal/noise ratio (SNR). Diminished SNR would require longer MR scan times, which detracts from a general goal of increasing scanner utilization. Thus, conventional designs and material choices for coil formers have presented a dilemma of optimizing either for PET or for MR imagery. One option has been to provide additional openings through the light structure of non-weight-bearing (anterior) formers. For example, anterior HNU formers in PET-MR environment may have additional holes to allow positrons to pass, but providing more openings is a technique that has only been used on non-supporting formers that are anterior to the patient. As PET relies upon near-simultaneous detection of two photons at opposite sides of a scintillator ring, having the additional holes only at the anterior side renders this option ineffective for actually enhancing the PET signal.